Back rest and arm rest height adjustments for chairs are known. Some of these adjustments are only settable at discrete locations within the range of movement of the back rest or arm rest. Other adjustments provide essentially an infinite number of positions in a range of movement of the chair component, but are cumbersome to use. For example, a threaded shaft provides non-discrete adjustment of a chair back rest but requires turning an adjustment nut through a large number of revolutions to move the back rest. Other arrangements use a slotted support having locking nuts which must be loosened and tightened for each adjustment of the chair.
U.S. Pat. No. 4,536,031 for a back support control mechanism for a chair or the like describes a mechanism used to adjust the position of a chair back rest through an essentially infinite number of positions within the range of movement of the back rest. According to the patent a channel member attached to the chair seat cooperates with a bar attached to the back rest to allow adjustment of the back rest position. To lock the position of the back rest a pressure pad attached to the bar is biased against the channel member by a movable lever.
The chair of the above patent, although workable, comprises numerous parts including a pressure pad and a spring system which substantially increase the manufacturing complexity and the risk of breakage of the parts.